The mechanical properties, dimensional stability, flexibility, weight, impact resistance and optical properties of polyester films offer advantages for their use in the manufacture of electronic or opto-electronic devices, such as electroluminescent (EL) display devices (particularly organic light emitting display (OLED) devices), electrophoretic displays (e-paper), photovoltaic (PV) cells and semiconductor devices (such as organic field effect transistors, thin film transistors and integrated circuits generally). The use of flexible polyester film as layer(s) in electronic devices allows the manufacture of such devices in a reel-to-reel process, thereby reducing cost.
A photovoltaic cell generally comprises a front-plane (or front-sheet); a front-side encapsulant material; the photoactive material on an electrode support substrate; a rear-side encapsulant; a rear back-plane (or back-sheet); and various components to collect and manage the electrical charge. Polyester films have been proposed in the manufacture of various layers in PV cells, for instance the front-plane, the back-plane, the electrode support layer(s). Photovoltaic modules, often consisting of many photovoltaic cells, are usually categorized according to the active photovoltaic materials used. These include crystalline silicon, gallium-arsenide (GaAs), amorphous silicon (a-Si), cadmium-telluride (CdTe), copper-indium-gallium-(di)selenide (CIGS), dye-sensitized or organic cells. Photovoltaic cells containing gallium-arsenide, amorphous silicon, cadmium-telluride, copper-indium-gallium-(di)selenide, dye-sensitized or conductive organic material are often referred to as thin-film photovoltaic cells (TFPV cells), which may or may not be flexible. Other thin-film silicon PV cells include protocrystalline, nanocrystalline (nc-Si or nc-Si:H) and black silicon PV cells. Thin-film photovoltaic cells are made by depositing one or more thin layers of photovoltaic material on a substrate, the thickness range of a thin layer varying from 1 or 2 nanometers to tens of micrometer, using a variety of deposition methods and a variety of substrates.
One type of PV cell which has attracted much research and development is the CIGS (copper-indium-gallium-(di)selenide) PV cell. While the efficiency of current CIGS PV cells is lower than that of crystalline silicon PV cells, the CIGS PV cells offer cost-benefits as a result of cheaper materials and fabrication methods. The active CIGS thin-film layer may be deposited directly on a substrate, typically steel or molybdenum-coated glass, in a polycrystalline form and this is typically achieved by vacuum sputtering. Manufacturing processes have been reported which involve a continuous, roll-to-roll process to deposit all layers of the CIGS solar cell on a stainless steel substrate. Alternative processes include (i) deposition of nanoparticles of the precursor materials on the substrate followed by sintering in situ; and (ii) electroplating. A typical CIGS PV cell comprises a supporting substrate on which is disposed a thin crystalline layer of photo-active CIGS material and further comprising a transparent polymeric layer over the CIGS material which may function for example as a protective layer or as a charge-collection layer. This multilayer composite is then encapsulated with a barrier material, which holds the composite together and serves to evacuate the air in the structure and to provide high resistance to gas and solvent permeation. The encapsulating barrier material is typically utilised in the form of a self-supporting film or sheet, which is then applied to the multilayer composite using thermal lamination techniques, typically under vacuum, as is known in the art. The encapsulated composite is then sandwiched between a front-plane and a back-plane, such that the transparent polymeric layer is disposed between the photo-active layer and the front-plane. The front-plane and/or back-plane is/are typically glass but may also be a polymeric material.
Other types of thin-film PV cell, as mentioned hereinabove, are similarly manufactured with a material to encapsulate the various functional layers, including the photo-active layers, of the PV cell.